Hash

SHA (Secure Hash Algorithm) is the name given to a series of cryptographic hash function families standardised by NIST under FIPS 180 and FIPS 202. Three generations exist with fundamentally different design lineages. SHA-1 (1995, FIPS 180-1) produces a 160-bit digest and is fully broken for collision resistance: the SHAttered attack (Google and CWI Amsterdam, 2017) produced a chosen-prefix collision — two different PDF files with identical SHA-1 hashes — using approximately 9.2 × 10^18 SHA-1 operations, within practical reach of well-resourced attackers. SHA-1 must not be used for any security purpose; it persists only in legacy Git object identifiers (SHA-1 is being phased out in Git’s object store in favour of SHA-256 under the sha256 object format) and in TOTP’s HMAC-SHA-1 inner construction (where collision resistance is not the relevant security property, but migration to SHA-256 variants is still recommended). SHA-2 (2001, FIPS 180-2 and subsequent revisions) is the Merkle-Damgård family that includes SHA-224, SHA-256, SHA-384, SHA-512, SHA-512/224, and SHA-512/256. SHA-256 and SHA-512 are the two variants in universal production use; the others serve niche roles. SHA-3 (2015, FIPS 202) is the Keccak sponge construction — structurally independent of SHA-2 — providing algorithm diversity and including fixed-output variants (SHA3-256, SHA3-512) and extendable output functions (SHAKE128, SHAKE256).